Dutch iris bulbs, of which there are many cultivars, are planted in the fall or early spring in preparation for their annual late spring or early summer bloom. Dutch iris bulbs are hardy in USDA zones 7 through 10 when mulched, and they can be dug up and overwintered indoors in colder climes. Transplant Dutch iris back into the ground in the spring after the last hard frost and when the ground can easily be worked.
Select a planting location with full sun exposure and nutrient-rich and easy draining soil. Choose high lands over low lands where water settles. Plant Dutch iris bulbs at intervals of 4 to 6 inches apart or a maximum of eight bulbs per square foot to avoid the need to dig and divide more than once every few years. Bury your iris bulbs so that the root plate of the bulb is 5 inches beneath the surface of the soil.
Water the iris bulbs well at planting time, and maintain evenly moist but not soaking wet soil around the bulbs throughout the growing season. Feed once or twice per year with a granular bulb fertilizer, according to label dosing directions, and water well after application.
Mulch over the iris bulb plantings in the fall after the dead top foliage has been cut away. Lay down a 2- to 3-inch deep blanket of compost, straw, leaf mold or other organic mulch to insulate the bulbs from cold and dry winter conditions. Water the soil lightly before mulching.
Store Dutch iris bulbs before planting or to overwinter in cool, dry, low light conditions where temperatures hover between 40 and 50 degrees Fahrenheit. Storage at 35 degrees Fahrenheit or colder will damage or kill the bulb. Use a clean cloth bag with good airflow or a container of clean dry sand to store the bulbs.